1. Field of the Invention
This invention relates to recovery of methane gas from subterranean coal seams. More particularly, the invention relates to a process wherein cryogenic liquid such as liquid nitrogen is utilized to increase the permeability of the portion of a coal seam penetrated by a wellbore.
2. Description of the Prior Art
Subterranean coal seams typically contain large volumes of methane. In the case of a mineable coal seam, it is desirable from a safety standpoint to produce as much of the methane as possible before beginning mining operations. In deeper coal seams, not amenable to conventional mining techniques, the methane constitutes a recoverable energy source which can be produced by conventional gas production methods.
Presently, methane is produced through wells drilled into the coal seams. Once a well is drilled and completed, it is common to treat the coal seam in order to stimulate the production of methane therefrom. One commonly used stimulation treatment involves hydraulically fracturing the coal seam much in the way other more conventional gas bearing formations are fractured. However, conventional hydraulic fracturing processes involve producing the fracturing fluid back through the wellbore, and this sometimes leaves permeability-reducing debris in the formation, and proppant sand often plugs horizontal wells. Gaseous fracturing fluids produce problems because of inability to adequately carry proppants and flow diverters, and foam fracturing fluids often leave flow-reducing residues. Also, sand or similar proppants sometimes produce back, plugging the well and/or damaging surface production equipment.
Another technique which has been proposed for stimulating a coal seam is one which is sometimes referred to as "cavity induced stimulation". In one form of that process, a wellbore is charged with a gas followed by a water slug. The well pressure is then reduced and the injected gas and water produce back and create a cavity by breaking up coal around the borehole face.
Cycling of the gas-water injection and blowdown followed by debris cleanout produces an enlarged wellbore cavity. However, this technique is not effective on many coal seams.
A variation of the cavity induced stimulation process in which liquid carbon dioxide is injected into the coal seam is described in U.S. Pat. No. 5,147,111 to Montgomery.
A method of stimulating water flow from a dry well is described in U.S. Pat. No. 4,534,413. That method involves alternate pressurization and depressurization of a well with liquid or gaseous nitrogen or carbon dioxide to fracture the borehole surface.
While the above-described processes have improved methane production in many cases, there remains a need for an improved stimulation process which is cheaper, safer and more effective than currently available processes.